The present invention relates to a process for coating laundry detergent or cleaning product tablets, which contain builder(s) and also, where appropriate, other laundry detergent and cleaning product ingredients.
Laundry detergent and cleaning product tablets have been widely described in the prior art and are enjoying increasing popularity among users owing to the ease of dosing. Tableted laundry detergents and cleaning products have a number of advantages over their powder-form counterparts: they are easier to dose and to handle, and have storage and transport advantages owing to their compact structure. Consequently, laundry detergent and cleaning product tablets have been described comprehensively in the patent literature as well. One problem which occurs again and again in connection with the use of detersive tablets is the inadequate disintegration and dissolution rate of the tablets under application conditions. Since tablets of sufficient stability, i.e., dimensional stability and fracture resistance, can be produced only by means of relatively high compressive pressures, there is severe compaction of the tablet constituents and, consequently, retarded disintegration of the tablet in the aqueous liquor, leading to excessively slow release of the active substances in the washing or cleaning operation. The retarded disintegration of the tablets also has the drawback that customary laundry detergent and cleaning product tablets cannot be rinsed in via the rinse-in compartment of household washing machines, since the tablets do not breakdown with sufficient rapidity into secondary particules small enough to be rinsed into the wash drum from said compartment. Another problem which occurs in particular with laundry detergent and cleaning product tablets is the friability of the tablets, or their often inadequate stability to abrasion. Thus, although it is possible to produce sufficiently fracture-stable, i.e., hard laundry detergent and cleaning product tablets, these tablets are often not up to the loads involved in packaging, transit and handling, i.e., falling stresses and frictional stresses, with the result that edge-fracture and abrasion phenomena may impair the appearance of the tablet or may even lead to complete destruction of the tablet structure.
To overcome the dichotomy between hardness, i.e., transport and handling stability, and the ready disintegration of the tablets, numerous approaches to solutions have been developed in the prior art. One approach, which is known in particular from the field of pharmacy and has expanded into the field of laundry detergent and cleaning product tablets, is the incorporation of certain disintegration aids, which facilitate the ingress of water or which, on ingress of water, swell, evolve gas, or exert a disintegrating effect in another form. Other proposed solutions from the patent literature describe the compression of premixes of defined particle sizes, the separation of certain ingredients from certain other ingredients, and the coating of individual ingredients, or of the whole tablet, with binders.
The coating of laundry detergent and cleaning product tablets is subject-matter of a number of patent applications.
For instance, European Patent Applications EP 846 754, EP 846 755 and EP 846 756 (Procter and Gamble) describe coated laundry detergent tablets comprising a xe2x80x9ccorexe2x80x9d comprising compacted particulate laundry detergent and cleaning product, and a xe2x80x9ccoatingxe2x80x9d, the coating materials used comprising dicarboxylic acids, especially adipic acid, which if desired comprise further ingredients, examples being disintegration aids.
Coated laundry detergent tablets are also subject-matter of European Patent Application EP 716 144 (Unilever). According to the details in that document, the hardness of the tablets may be intensified by means of a xe2x80x9ccoatingxe2x80x9d without detracting from the disintegration and dissolution times. Coating agents specified are film-forming substances, especially copolymers of acrylic acid and maleic acid, or sugars.
The coating of the tablets is advantageous for the strength, the reduction of abrasion and dust, edge stability, storage stability, visual impression, and the sensory quality on handling by the user. The coating ought to envelop the laundry detergent and cleaning product tablet. In order to do so, the coating material, which is used in the form alternatively of a melt, solution or dispersion, must be applied with the maximum of uniformity and targetedness.
The processes known from the prior art have the drawback that the application of the coating by means of spraying or dipping methods imposes particular requirements on the properties of the coating material, in particular on its viscosity.
The object on which the present invention was based was to provide a process for coating laundry detergent and cleaning product tablets which allows both the top and bottom faces and also the sides to be coated, and in the case of which it ought also to be possible to apply partial coatings. Owing to the tablets"" inherent sensitivity to mechanical loads, a further object was to provide a process for coating such tablets in which the tablets are exposed only to a very low mechanical load.